Development of a protocol that could invoke specific suppression of an undesired immune response, while sparing normal immune competence, would be of great clinical value. This report demonstrates that multiple infusions of splenocytes sensitized in vivo to sheep red blood cells (SRBC) and photoinactivated in vitro with 8-methoxypsoralen and ultraviolet A light can render a syngeneic recipient selectively unresponsive to subsequent challenge with this antigen. Mice treated in this fashion did not develop a T cell-mediated delayed type hypersensitivity (DTH) reaction to SRBC. In contrast, control mice exposed to nonimmune splenocytes pretreated in an identical manner developed a normal DTH response to SRBC, thereby demonstrating that drug and light in the absence of effector T cells were not suppressive. Inhibition of the DTH response was antigen specific, since animals rendered unresponsive to SRBC developed a normal DTH response to chicken red blood cells. Cell transfer experiments demonstrated that unprimed recipients of splenocytes from mice rendered unresponsive to SRBC could not mount a DTH reaction when challenged. Moreover, this procedure can also suppress established immunity to that antigen. The use of photoinactivated syngeneic antigen-reactive effector cells as immunosuppression agents suggests that this method may be clinically useful in inhibiting pathogenic antigen-specific immunologic reactions.